Bipolar integrated circuits, for example, bipolar analog integrated circuits comprise a large number of resistors. The resistors of the bipolar integrated circuits typically are made by diffusion processes.
A typical diffusion resistor is shown in FIG. 1. In FIG. 1, the diffusion resistors 10 are comprised of the island areas 10a diffused on a substrate 10b and resistor areas 10c diffused on the island areas 10a. The diffusion resistors 10 are separated from other diffusion resistors 10 or other circuit elements by isolation areas 10d diffused between the island areas 10a. The island area 10a and the resistor area 10c correspond to the collector area and the base area of typical transistors constructed on the integrated circuit by diffusion processes. The diffusion resistors 10 are connected in series by coupling the resistor areas to the adjacent resistor areas.
Bipolar analog integrated circuits for audio apparatus are required to have less distortion. However, diffusion resistors are insufficient in respect to distortion, compared to other types of resistors. Therefore, some proposals have been made for reducing distortion of circuits having diffusion resistors.
For example, Japanese magazine "Nikkei Electronics", Issue of Nov. 9, 1981, pp. 122-136, reports an article regarding a diffusion resistor circuit. FIG. 3 on page 128 of the article entitled "Integrated Circuits dedicated for Noise Suppression Circuit of Audio Apparatus" discloses a diffusion resistor circuit with an improved distortion.
An equivalent circuit to that of the above article is shown in FIG. 2 of this application for the sake of convenience. As shown in FIG. 2, three diffusion resistors 10 are connected in series. In each diffusion resistor 10, one end terminal 10e of each resistor area 10c is coupled to an island area 10a thereof. As a result, the potential of the one end terminal 10e of the resistor area 10c is equalized to the potential of the island area 10a in each diffusion resistor 10. The diffusion resistors 10 are coupled in series to obtain a desired value of resistance. In the series circuit, the island areas 10a coupled to the one end terminals 10e of the resistor areas 10c are coupled to the other end terminals 10f of adjacent diffusion resistors 10. Therefore, the potentials of the island areas 10a are applied to the same end terminals 10e of the resistor areas 10c.
The article says that the diffusion resistor circuit reduces distortion lower than the distortion in prior circuits by one order. However, the reduction of the distortion is still insufficient for higher grade audio apparatus. This is because the distortion generated by the resistor areas 10c is accumulated in the series circuit.
A report entitled "Application of Poly-silicon Resistors to Analogue Integrated Circuit" in "Electronics and Communication Engineers Institute (Japan), Technical Study Report", SSD 79-48, issued in 1978, reports another example for reducing distortion in integrated circuits having diffusion resistors. FIG. 14 in this Report, on page 55, shows a graph of total distortion to output voltage response characteristics. In FIG. 14, the total distortion in the case of the poly-silicon resistor is reduced greatly in comparison to distortion in the case of other diffusion resistors. However, the report is limited to an application for an inverse phase feedback amplifier, as shown in FIG. 13, on page 59 of the report. That is, the poly-silicon resistor has a typical resistance value like other diffusion resistors. Further, the poly-silicon resistor has some drawbacks in that it requires many steps to manufacture and is high in cost compared to other diffusion resistors.